1. Field of the Invention
The present invention relates to a communication system, a control method thereof, a terminal station, and a computer-readable storage medium.
2. Description of the Related Art
The need for wirelessly connecting not only computer peripheral devices but also consumer devices such as a digital still camera, cellular phone, and music player is growing.
A UWB (Ultra Wide Band) wireless method is a wireless communication method for implementing a WPAN (Wireless Personal Area Network) with a high data transfer rate. For example, a UWB method complying with technical specifications defined by the FCC (Federal Communication Commission) in 2002 uses a very wide frequency band of 3.1 to 10.6 GHz. Additionally, the ecma-368 standard is developed as technical specifications concerning the physical layer and the media access function of WPAN using UWB (ecma, High Rate Ultra Wideband PHY and MAC Standard, December, 2005). The UWB physical layer defined by this standard uses an OFDM (multi-band OFDM: Orthogonal Frequency Division Multiplexing) method. In the OFDM method, an OFDM modulated signal having a bandwidth of 528 MHz causes frequency-hopping. This method implements a WPAN system having a data transmission rate of 480 Mbps at maximum.
The ecma-368 standard assumes USB, IEEE1394, or IP protocol as the upper-level protocol. The USB standard setting body has also developed the WUSB (Wireless USB) standard that is a wireless USB connection standard (USB-IF, “Wireless Universal Serial Bus, Revision 1.0”, May, 2005).
USB by wired connection will be explained. FIG. 9 is a block diagram showing an example of a system arrangement using USB. In this system arrangement, a USB host 71 serving as a control device is connected to one or more USB devices 72 serving as peripheral devices via USB cables 73 to form a star network. The USB can also form a tree-shaped topology by using a branching device called a hub 74.
Conventionally, the USB has been developed as an interface to connect a personal computer to a peripheral device. A personal computer serves as a host, and a peripheral device such as a printer or a scanner serves as a device in general. In recent years, however, many devices other than personal computers are also getting the USB host function. An example is a direct-print method of connecting a digital still camera (DSC) to a printer and causing it to print an image from the digital still camera. In this case, the printer operates as a host, and the digital still camera operates as a device. Hence, the printer having the direct-print function has both a USB device port which connects the printer serving as a device to a personal computer and a USB host port which connects the printer serving as a host to the digital still camera.
FIG. 10 is a block diagram showing an example of the system arrangement of the direct-print method. A printer 82 includes both a USB device port 85 and a USB host port 86. The USB device port 85 of the printer 82 is connected to a personal computer 81 via a USB cable 83. In this case, the personal computer 81 operates as the host of the printer 82.
To connect the printer 82 to a digital still camera 84 to perform direct-print, the user connects the digital still camera 84 to the USB host port 86 of the printer 82 using a USB cable. At this time, the printer 82 operates as the host of the digital still camera 84.
The operation of the WUSB will be described next. FIG. 11 is a block diagram showing an example of a system arrangement using WUSB. In this system arrangement, a WUSB host 91 is connected to WUSB devices 93 to 95 via WUSB wireless links 92. An example of the WUSB host 91 is a personal computer. Examples of the WUSB devices 93 to 95 are a printer, scanner, hard disk drive, and digital still camera. The WUSB host 91 and the WUSB devices 93 to 95 exchange wireless frames using the WUSB wireless links 92, thereby executing data transfer.
FIG. 12 is a timing chart showing the timings of wireless frames transmitted via the WUSB wireless links 92 shown in FIG. 11. Note that the timings of wireless frames shown in FIG. 12 conform to a protocol defined by the WUSB standard.
Data transfer in the WUSB starts at a broadcast frame 1001 transmitted from the WUSB host 91. The broadcast frame 1001 includes a unique identifier to identify the WUSB host 91. The identifier is a value uniquely assigned to the WUSB host 91, and is called a CHID (Connection Host ID) in the WUSB standard. This identifier is registered in advance in the WUSB devices 93 to 95 connectable to the WUSB host 91 having the identifier. A method of registering such a WUSB host identifier is defined by the Association Model that is an associated standard of WUSB. A detailed description of the identifier registration method will be omitted. Each of the WUSB devices 93 to 95 detects the identifier, thereby searching for the connectable WUSB host 91.
After the WUSB host 91 has transmitted the broadcast frame 1001, the WUSB host 91 transfers data to the WUSB devices 93 to 95. A wireless frame 1002 is transferred from the WUSB host 91 to the WUSB device 93. A wireless frame 1003 is transferred from the WUSB host 91 to the WUSB device 94. A wireless frame 1004 is transferred from the WUSB host 91 to the WUSB device 95. The broadcast frame 1001 includes pieces of time information that designate time slots to transmit the wireless frames 1002, 1003, and 1004. Upon receiving the broadcast frame 1001, each of the WUSB devices 93 to 95 analyzes the time information, and receives the wireless frame designated to it at an appropriate timing.
Next, each of the WUSB devices 93 to 95 transfers data to the WUSB host 91. A wireless frame 1005 is transferred from the WUSB device 93 to the WUSB host 91. A wireless frame 1006 is transferred from the WUSB device 94 to the WUSB host 91. A wireless frame 1007 is transferred from the WUSB device 95 to the WUSB host 91. The broadcast frame 1001 also includes pieces of time information that designate time slots to transmit the wireless frames 1005, 1006, and 1007. Upon receiving the broadcast frame 1001, each of the WUSB devices 93 to 95 analyzes the time information, and transmits the wireless frame at an appropriate timing. This enables to prevent collision of wireless frames. The WUSB protocol thus controls to make all WUSB devices smoothly communicate by referring to the identifier and frame transmission/reception timing information included in the broadcast frame transmitted from the WUSB host.
The WUSB host ensures a band using the DRP (Distributed Reservation Protocol) method of the ecma-368 standard to communicate with each WUSB device. FIG. 13 is a view showing a wireless frame of a MAC layer defined by the ecma-368 standard. The wireless frame includes a plurality of super frames 1101 to 1103 repeatedly. One super frame has a length of 65.536 ms, and is divided into 256 slots called media access slots (MASs) 1105 at an interface of 256 μs. A period called a beacon period (BP) 1104 and having a variable length is located at the start of the super frame. The beacon period 1104 can use 32 MASs at maximum. The beacon period 1104 has a role of transmitting various kinds of control information. The WUSB host or WUSB device announces the control information of its own to other devices during, e.g., this period. The media access slots 1105 except the beacon period 1104 are assigned to data transmission. The ecma-368 standard defines two data transmission methods: the PCA (Prioritized Contention Access) method and the DRP method.
In the PCA method, data transmission can freely be done outside the beacon period 1104 and DRP-reserved media access slots 1105. To the contrary, the DRP method reserves in advance MASs to be used for transmission (this information is sent to other devices by a beacon in the beacon period 1104), and performs data transmission only in the determined time period. In FIG. 13, the WUSB host ensures the band necessary for communication with WUSB devices as DRPs #1 1106 to #3 1108 each including several media access slots 1105.
Each of the DRPs #1 1106 to #3 1108 shown in FIG. 13 includes information called MMCs (Micro-scheduled Management Commands) 1109 to 1111. An MMC holds information to identify the WUSB host and a time to the next MMC in a sequence (i.e., link). The series of links ensures the transmission band for data communication between the WUSB host and a WUSB device.
As described above, the WUSB standard defines the DRP method as a method to be used by the WUSB host and the WUSB device to ensure the band for data communication. The standard also defines to make WUSB hosts adjust collision upon band acquisition using a perfect distributed control method.
Techniques of solving collision in band acquisition have been proposed conventionally. For example, a technique is known which, if an unused communication band is short, reduces the already set communication bands of other communication connections, thereby ensuring the necessary band (Japanese Patent Laid-Open No. 07-099526). There is also known another technique which, if band ensuring has failed due to a shortage of band, reduces the band ensured by another device connected to the transmission path, and ensures the freed band (Japanese Patent Laid-Open No. 2005-012260).
The WUSB is also usable for direct-print. FIG. 14 is a block diagram showing an example of the system arrangement of the direct-print method using the WUSB. In this system arrangement, a printer 1201 serving as a WUSB host is connected to a digital still camera 1202 serving as a WUSB device by wireless direct-print 1203. This arrangement allows the printer 1201 to print an image from the digital still camera 1202.
Wireless frame exchange in direct-print also conforms to the operation of the WUSB protocol shown in FIGS. 12 and 13.
In some product forms, a USB device incorporates an adapter function as shown in FIG. 15. In this system arrangement, a printer 1304 having a wireless hub function is connected to a personal computer 1301 via a USB cable 1303. FIG. 16 is a block diagram showing an example of the internal arrangement of the printer 1304 shown in FIG. 15. The printer 1304 incorporates a wired USB hub 1403. A printer function 1401 and an adapter function 1402 are connected to the hub 1403.
A printer 1504 shown in FIG. 17 performs wireless communication with a personal computer 1501. The printer 1504 also has a wireless direct-print function for a digital still camera. The printer 1504 operating as a WUSB device is wirelessly connected, via a WUSB wireless link 1502, to the personal computer 1501 operating as a WUSB host. The printer 1504 can also execute wireless direct-print 1506 with a digital still camera 1505.
FIG. 18 is a block diagram showing an example of the internal arrangement of the printer 1504 shown in FIG. 17. A wireless function switching unit 1601 selects a device function unit 1602 or a direct-print function unit 1603, and is connected to a WUSB wireless unit 1604. A print function switching unit 1605 selects the device function unit 1602 or the device function unit 1602, and is connected to a print function unit 1606.
If the printer 1504 operates as a WUSB device of the personal computer 1501, the wireless function switching unit 1601 selects the device function unit 1602.
If the printer 1504 executes wireless direct-print, the wireless function switching unit 1601 and the print function switching unit 1605 select the direct-print function unit 1603. The print function unit 1606 is thus connected to the digital still camera 1505 via the direct-print function unit 1603.
As described above, there can be a device having both the WUSB device function and the WUSB host function. Such a device is called a DRD (Dual-Role Device). If a DRD is, e.g., a printer, it operates as both a peripheral device of a personal computer and a host that controls wireless direct-print.
The WUSB standard defines a band acquisition method using the DRP method and an obligation to free an unused band. However, there is no specific definition about the timing when a WUSB host should ensure a band for a WUSB device by the DRP method. For this reason, it may be impossible to acquire a band when, for example, a DRD operating as a WUSB device is going to switch the operation to that of a WUSB host. A detailed example of the case will be described with reference to FIG. 17.
The personal computer 1501 operating as a WUSB host acquires a communication band using the DRP method when connecting to the printer 1504 operating as a WUSB device, and communicates with the printer 1504 using the communication band. Assume that in this state, the printer 1504 and the digital still camera 1505 establish connection to perform wireless direct-print. In this case, the printer 1504 operating as a WUSB device changes to a WUSB host by the DRD function of its own, and newly ensures a communication band for the digital still camera 1505. This communication band cannot be acquired in advance. It can be acquired only when the printer 1504 actually starts connection to the digital still camera 1505.
Band acquisition by a WUSB host is managed by each WUSB host using distributed control. If an acquisition target band collides with another WUSB host, the WUSB hosts adjust it. For example, when a number of WUSB hosts and WUSB devices exist in the periphery, and they have already occupied the bands, negotiations with the other WUSB hosts start to ask for band assignment. However, these negotiations need not always lead to band assignment. That is, even when a printer having a DRD function changes to a WUSB host for wireless direct-print, it may be unable to ensure a band.